Monday, February 10, 2014

Climate models robustly predicted the opposite of what has caused the record cold US weather

An article published today "Winter weirdness: Is Arctic warming to blame?" notes "this winter has brought unseasonable warmth to Alaska, frigid temperatures to much of the Eastern US, and more drought to California. The jury is still out on whether a warmer Arctic is behind the extreme weather." "When persistent weather patterns have brought drought or heat waves or repeated invasions of cold air to usually mild locations in winter, these links to the Arctic have become a go-to explanation among many commentators and policymakers."

Of particular note, the article points out that climate models actually predicted the opposite pattern to occur with the jet stream drawn north, with fewer jet stream dips, and no change in jet stream blocking:

"These [modeling] studies suggest that a warming Arctic will draw the jet stream's average track north.Blocking patterns will decrease. Moreover, the models indicate no "robust" decrease in the jet stream's speed, notes Elizabeth Barnes, a climate scientist at Colorado State University in Fort Collins who focuses on the jet stream's behavior and the factors affecting it. To be sure, the models could be wrong, she acknowledges. But when different teams with different models converge on the same answer, that inspires more confidence in the result."

Furthermore, the article notes "Long-term swings in Atlantic sea-surface temperatures, known as the [natural] Atlantic multidecadal oscillation, appear to have the same effect on the jet stream's meanders and blocking patterns that Arctic warming and sea ice are purported to have.

When the AMO enters its warm phase – its condition since the 1990s – the jet stream tends to weaken and buckle. Blocking patterns increase, and colder temperatures prevail at mid-latitudes.

"This also supports the colder winters of recent years," Magnusdottir says, adding that the results seem robust, since they show up in real-world data as well as in computer simulations."

This winter has brought unseasonable warmth to Alaska, frigid temperatures to much of the Eastern US, and more drought to California. The jury is still out on whether a warmer Arctic is behind the extreme weather.

Ice from her breath forms around the face of a woman in Minneapolis. Temperatures plunged in early January as the polar vortex enveloped much of the Eastern US.

For Alaskans who have basked in record warmth, Atlantans who abandoned cars during a January snowstorm, or Californians enduring drought, this winter's extremes have been nothing if not memorable.

Drought or unusual warmth is in sync with the effects that climate scientists expect from global warming. But what about wintertime invasions of Arctic air into the US Deep South or into China, where, a new study indicates, record cold events became more frequent over the past 10 to 20 years?

For some climate scientists, January's extremes and the atmospheric patterns that nurtured and sustained them are fresh bits of information to apply to these intriguing questions: Has global warming's effect on the Arctic set the stage for persistent weather patterns that lead to extremes? If so, is the decline in Arctic sea ice the stage manager for the wintry events?

Nearly two years ago, two researchers identified atmospheric features that appear to tie a warming Arctic to mid-latitude weather extremes. Since then, when persistent weather patterns have brought drought or heat waves or repeated invasions of cold air to usually mild locations in winter, these links to the Arctic have become a go-to explanation among many commentators and policymakers.

Researchers working on the issue, however, caution that while they are uncovering intriguing hints of this tie-in,Ice from her breath forms around the face of a woman in Minneapolis. Temperatures plunged in early January as the polar vortex enveloped much of the Eastern US.

Nor is it merely an arcane debating point. If the proposition holds up, in principle it could help improve seasonal forecasts for temperature and precipitation.

"That would have huge economic implications," says Cecilia Bitz, a climate scientist at the University of Washington in Seattle who specializes in the role that Arctic sea ice plays in the climate system.

"We are in the same situation," says Dr. Francis, who, along with colleague Stephen Vavrus at the University of Wisconsin-Madison, published the hypothesis about Arctic links to weather extremes in the journal Geophysical Research Letters in March 2012.

The duo starts by noting that warming in the Arctic has occurred at two to three times the pace of warming for the rest of the Northern Hemisphere, reducing the temperature difference between the Arctic and mid-latitudes. As the contrast shrinks, the jet stream slows.

The jet stream is a river of high-altitude winds that steers and spawns storms and in effect serves as an atmospheric boundary for cold Arctic air. A slower jet stream produces longer meanders, the pair says. This would bring cold air farther south and warm air farther north than otherwise would be the case.

At the same time, the two say, circulation patterns that block the migration of these meanders appear more frequently and persist. This would cause specific weather patterns to either creep along or dally for days to weeks.

Other researchers have connected a warming Arctic to weather extremes in specific regions. At a two-day workshop on the topic at the University of Maryland in College Park last September, climate scientist James Screen of the University of Exeter in England pointed to studies that link shrinking sea ice to cold winters in North America and Eurasia, wet summers in Europe, extreme rainfall in the Mediterranean, and the behavior of the East Asian monsoon.

But Dr. Screen also cautioned against claiming too much for the connections – at least at this point. An apparent link doesn't say much about cause and effect, he and others noted.

The case for the link between the shrinking extent of Arctic sea ice and cold winters in Eurasia was the strongest, he offered. Even here, however, the influence is barely detectable and often not statistically significant.

The caution that he and others express centers in no small part on the radically different effects that Francis and Dr. Vavrus see compared with a decade's worth of modeling studies.

These studies suggest that a warming Arctic will draw the jet stream's average track north. Blocking patterns will decrease. Moreover, the models indicate no "robust" decrease in the jet stream's speed, notes Elizabeth Barnes, a climate scientist at Colorado State University in Fort Collins who focuses on the jet stream's behavior and the factors affecting it.

To be sure, the models could be wrong, she acknowledges. But when different teams with different models converge on the same answer, that inspires more confidence in the result.The scientists' caution also hinges on the relatively short time that the Arctic has undergone rapid warming, Francis adds, noting that it has occurred only in the past 10 or 15 years. Model projections can help, but "it's going to take maybe another decade" before any signs of an Arctic influence can be confidently detected, she says.

Francis and colleagues also have looked specifically at sea ice loss and winter weather extremes, finding a link.

In effect, the ever-shrinking amount of sea ice in the fall makes more moisture available for snow. Increased snowfall over the North American or Eurasian Arctic earlier in the fall-winter season cools the air more quickly than a later snowfall would.

This more southerly mass of cold air affects pressure patterns in ways that boost the likelihood of blocking patterns and cold snaps.

This idea appears to draw some support from modeling work that was published last August and conducted by Yannick Peings and Gudrun Magnusdottir of the University of California, Irvine.

When they plugged the average Arctic sea ice decline between 2007 and 2012 into the model, it yielded cold conditions at mid-latitudes, mainly over Asia. The model also produced some increase in the depth of the jet stream's meanders, although the statistical significance was small. All this tended to take place in February.

But by 2090, according to their model projection, the Arctic warmed so much that wintertime cold extremes at mid-latitudes remained only as frequent as they were in 2010.

Further work by Dr. Magnusdottir and colleagues has added another wrinkle. Long-term swings in Atlantic sea-surface temperatures, known as the [natural] Atlantic multidecadal oscillation, appear to have the same effect on the jet stream's meanders and blocking patterns that Arctic warming and sea ice are purported to have.

When the AMO enters its warm phase – its condition since the 1990s – the jet stream tends to weaken and buckle. Blocking patterns increase, and colder temperatures prevail at mid-latitudes.

"This also supports the colder winters of recent years," Magnusdottir says, adding that the results seem robust, since they show up in real-world data as well as in computer simulations.Others looking for predicted circulation patterns have a hard time finding anything. In a study published last month in Geophysical Research Letters, Dr. Barnes of Colorado State and colleagues used three independent approaches to identifying blocking patterns in climate data. They reported finding trends in isolated regions and at specific periods, but no trend was significant and common to all three approaches.

"Blocking is so variable from year to year that there's no evidence that anything out of the ordinary is occurring," Barnes says. Nor has she found trends in changes to the jet stream's meanders or travel time around the hemisphere.

The evidence may be masked for now by the "noise" of natural variability. And other factors affect the jet stream. Perhaps "you have a tug of war going on" between the influence of sea ice and something else, Barnes says.

For her part, Francis acknowledges that for now, several of the links tying Arctic warming to patterns that foster bouts of extreme weather are still weak. On the other hand, no one is finding that the meanders in the jet stream are getting any smaller, she adds.

"Everybody's seeing a hint that this is happening," she says, "but we can't say it's definitely happening yet."

JC message to Jennifer Francis: I’ve found that your credibility is reduced and your own motivations are questioned when you attack the motives of another scientist [Barnes], particularly a young scientist without any apparent agenda beyond doing good science and advancing her academic career. The high ground is a much better place to be, and not just in a hurricane.

11 comments:

Prof Mat Collins, an internationally acknowledged expert on climate computer models and forecasts, insists recent storms are not linked to global warming. The ‘coordinating lead author’ for the UN’s Panel on Climate Change explained that the storms were driven by the jet stream which has been “stuck further south than usual”. He claimed, “There is no evidence global warming can cause the jet stream to get stuck in this way and if this is due to climate change, it is certainly outside our knowledge.” As usual Prof Collins was at odds with Met Office chief scientist Dame Julia Slingo who in November predicted, “drier-than-normal conditions across the UK this winter’. The real problem is not our unpredictable winter weather but that thousands of farmers and householders were betrayed by the misconceived policies of EU and UK greens.

The record lake-effect snow in the Buffalo area (November 18-20, 2014) has been said to be caused by global warming...because less ice cover due to warming means more precipitation events in the form of snow...or so it's claimed.

But scientists have, in the past, concluded that global warming causes reduced lake-effect snow, not increases in lake-effect snow.

------------http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-12-064.1"Trend Reversal in Lake Michigan Contribution to Snowfall"A general increase in LCS [lake-contribution snowfall] from the early 1920s to the 1950–80 period at locations typically downwind of the lake was found. Thereafter, LCS decreased through the early 2000s, indicating a distinct trend reversal that is not reported by earlier studies. The reasons for this reversal are unclear. The reversal is consistent with observed increasing minimum temperatures during winter months after the 1970s, however.-------------http://web2.geo.msu.edu/glra/PDF_files/Regional%20Summary/03-F_lake_effect.pdfThus, there may be little change in the frequency of heavy lake-effect snow in the Lake Superior snowbelt and a substantial decrease in the southern Lake Michigan and Lake Erie snowbelts. [A]ir-temperature [warming] was found to be the primary determining factor in reducing the frequency of heavy lake-effect events in this study...[A]nticipated regional impacts of climate change on lake-effect snow patterns – suggest[] almost no change [in lake-effect snowfall] in the northernmost belts but approximately a 50% decrease in southernmost belts.-------------http://www.sciencedirect.com/science/article/pii/S0380133002706035Surface conditions favorable for heavy lake-effect snow decreased in frequency by 50% and 90% for the HadCM2 and CGCM1, respectively, by the late 21st Century. This reduction was due almost entirely to a decrease in the number of occurrences of surface air temperature in the range of −10 to 0°C, which in turn was the result of an increase in average winter air temperatures.

Here's another one, from 1971, that says that global cooling (which was occurring during that period) contributed to increased lake-effect snowfall during the 1940s to 1970s, and global warming (during the 1920s and 1930s) contributed to decreased lake-effect snowfall.

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0477%281970%29051%3C0403%3ALESTTL%3E2.0.CO%3B2Evidence suggests that lake effect snowfall has significantly increased during the past several decades, particularly in Southern Michigan and Northern Indiana. While the observed changes cannot be definitively ascribed to any single factor, it seems likely that a general cooling of winter temperatures may be partially responsible for this climatic change. [M]any of the snowfall time-series curves for the lake stations show downward trends during the 1920’s and 1930’s, at the height of the recent warm period, and the more recent snowfall increase has coincided with a general world-wide cooling which has occurred in the last several decades [1940s-1970s]. Recent evidence derived from [isotope] analysis of ice core samples on the Greenland ice cap indicates a continuance of this cooling trend for another 20 or 30 years [through the 1990s].